As means of aiding in the dewatering of sludge, the practice of using a high molecular weight flocculant and fibers such as, for example, fibers of pulp or sawdust in their simply mixed manner has been known to the art. This mixed auxiliary is inferior to an inorganic flocculant in terms of water content in the cake formed during the dewatering operation and ease of separation of the cake from the filter cloth. It further has the disadvantage that the use of the mixed auxiliary entails much time and labor, necessitates provision of large facilities for storage and transportation, and fails to manifest a stable effect.